The exchange rate of the amide protons in staphylococcal nuclease H124L is being investigated. The kinetics of hydrogen exchange between solvent and protein are a powerful probe of the structure, dynamics and stability of proteins in solution, since even under conditions such that the native state is much more stable than the denatured state, the hydrogens on all but a few core peptide nitrogens will exchange with water. In interpreting the kinetics, the Linderstrom-Lang model of NH-exchange is applied, and an EX2 exchange mechanism is assumed. The opening rate is determined (the rate of conformational change from the "closed" to the "open" state from which the exchange is thought to take place) at a number of different pressures, and by calculating the dependence of the equilibrium position between the open and closed states on pressure, one can determine the change in partial molar volumes between the two states.